A fiber-rich diet outperformed metformin for blood sugar and weight in a clinical trial

A head-to-head study asked whether changing diet could reshape the gut microbiome and affect two major treatment goals in type 2 diabetes.

Study: A high-fiber diet altered gut microbiota, improved glycemic control, and reduced body weight in people with newly diagnosed type 2 diabetes and obesity. Image Credit: Tatjana Baibakova / Shutterstock.com

Researchers reporting in Frontiers in Microbiology compared a high-fiber dietary program with standard metformin alone in adults living with diabesity, the combination of type 2 diabetes and obesity.

Why fiber and the gut microbiome matter in diabesity

Diabesity is a commonly understood but infrequently used medical term for the overlap between type 2 diabetes mellitus and obesity. As both conditions continue to rise worldwide, finding better ways to improve glucose control and reduce excess body fat has become a major research priority.

At the same time, studies of the gut microbiome have shown that these microbes can influence metabolism. Beneficial bacteria break down fiber into short-chain fatty acids, or SCFAs, which can trigger the release of glucagon-like peptide-1, known as GLP-1. That hormone supports insulin sensitivity and increases satiety, both of which can help regulate blood sugar.

In contrast, an imbalanced microbiome, or dysbiosis, has been associated with microbial products such as lipopolysaccharide and hydrogen sulfide, which may drive inflammation or disrupt metabolic signaling, including GLP-1 pathways.

Until now, no trial had directly compared metformin, the usual first-choice drug for type 2 diabetes, with a high-fiber diet designed to support the microbiome.

Trial compares fiber intervention with metformin monotherapy

To address that gap, the researchers ran an 84-day, open-label head-to-head trial comparing metformin with a standardized high-fiber dietary intervention.

The study took place at Henan Provincial People's Hospital in China and enrolled 48 adults between the ages of 18 and 70 who had newly diagnosed type 2 diabetes. Eligible participants had baseline glycated hemoglobin, or HbA1c, between 6.5% and less than 9.0%, along with a body mass index of at least 24 kg/m².

Forty-six participants completed the study. After a two-week run-in period, they were assigned either to a high-fiber diet without medication or to metformin alone. Two people in the fiber group dropped out, leaving 34 who finished in that arm.

The fiber regimen included soluble and insoluble fibers such as inulin and resistant dextrin. The main outcomes were HbA1c and BMI at the end of the study, with analyses adjusted for baseline values. Secondary measures included continuous glucose monitoring, meal tolerance testing, and dual-energy X-ray absorptiometry.

Researchers also analyzed serial stool samples using shotgun metagenomics and metabolite testing to track changes in the gut microbiome over time.

Fiber improved blood sugar, weight, and gut microbial function

By the end of the trial, baseline-adjusted HbA1c was lower in the fiber group than in the metformin group, at 6.3% versus 6.7%. The fiber group also had a lower adjusted mean BMI, 28.4 kg/m² compared with 30.2 kg/m².

People in the diet group also ate less energy and consumed smaller amounts of several macronutrients during the intervention, which may have contributed to the weight-related differences. Continuous glucose monitoring showed less glycemic variability in the fiber group, along with lower total body fat and triglyceride levels at the end of the study.

Microbiome analysis showed that the high-fiber diet changed community structure within 28 days, increasing bacteria such as Bifidobacterium and Lactobacillus while reducing Bilophila wadsworthia, a microbe associated with chronic inflammation. In the metformin group, no major shift in overall microbial community structure was seen during the trial, although smaller changes in specific microbes could not be excluded.

Functional analysis found higher levels of the acetate-related gene formate-tetrahydrofolate ligase (fhs) at days 28 and 56 in the fiber group, along with lower levels of the hydrogen sulfide–related genes sarD and dsr by the end of the study. Fecal hydrogen sulfide concentrations were also lower than those in the metformin group at days 28 and 56, although the difference was no longer significant by the final assessment.

Promising results, but larger trials are still needed

Overall, the study suggests that a targeted high-fiber diet produced greater improvements in HbA1c and BMI than metformin in people with newly diagnosed diabesity.

Still, the authors noted important limits. The trial was single-center, only 46 participants completed treatment, and just 12 were in the metformin group. The short duration and uneven group sizes reduce confidence in how broadly the findings can be applied.

The study also did not measure GLP-1 directly, so it could not confirm whether the metabolic effects were driven by changes in SCFAs or hydrogen sulfide.

Baseline microbial patterns differed between participants who did and did not show increased fecal acetate, but these observations came from small exploratory subgroups. Larger independent studies will be needed to see whether these microbiome features can predict who benefits most from a high-fiber diet.